Re: A China-Sumer connection?

From: Martyn Harrison (nospam_at_spammers.of.the.world.unite)
Date: 02/25/05 

Date: Fri, 25 Feb 2005 23:59:39 GMT

Apparently on date Fri, 25 Feb 2005 21:02:29 +1000, Jacques Guy
<jguy@alphalink.com.au> said:

>Martyn Harrison wrote:
>
>> Jupiter goes from being near the earth
>> (and therefore brighter) to hugely distant (and hard to see) in a few months.
>
>That cannot be right. Jupiter is 5.2 astronomical units (a.u.) from the
>Sun, and
>the Earth is 1 a.u. from it (by the very definition of "astronomical
>unit").

I could have put that a lot better. The relative positions of planet and sun
relative to an observer on earth alter quickly in a few weeks / months. The
effect is mostly because when Sun - Earth - Jupiter are in a line, the whole
face of Jupiter is reflecting sunlight back to us over the shortest distance.
As we race ahead of Jupiter, it moves away from us and also changes phase as we
can see less of the sunlit side.

The point I was trying to make was that being even only a few months out of
sync makes a large difference. 59 years instead of 60 is more than just "almost
the same as the next year", it's quite different in relative positions to an
observer:

- A twelve year cycle makes sense, Jupiter looks about right after one complete
cycle.

- A couple of cycles starts to look wrong, if you have a decent way to define
it (e.g. at a given season, say a few weeks in the year, which of a dozen
constellations around the ecliptic it happens to be in through that season.)

- Sixty years is completely out of sync, a worse piece of observational
astronomy than 12 years - as it is obviously back on the cycle after 59 years,
and nowhere near on 60. Beyond that it's probably an understanding of what
causes these cycles and measuring the actual period of orbit around the sun, I
reckon.

>You must have had Venus in mind. Venus is 0.72 a.u. from the Sun. So,
>when closest to the Earth, it is 0.28 a.u. away but it is 1.72 a.u. away
>when furthest. Now _that_ is a huge difference indeed.

Which underlines my poor choice of wording. Venus is, of course, pretty much
invisible when it is nearest us because, being closer to the sun, obviously the
sun can't reflect off it in our direction. Almost the reverse situation.

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